ARM: Add an earlyprintk debug console

This patch allows an earlyprintk console if CONFIG_DEBUG_LL is enabled,
using the printch asm function.

The patch is based on the original work by Sascha Hauer.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Pavel Machek <pavel@ucw.cz>

arch/arm/Kconfig.debug

@@ -71,6 +71,14 @@ config DEBUG_LL
 	  in the kernel.  This is helpful if you are debugging code that
 	  executes before the console is initialized.
 
+config EARLY_PRINTK
+	bool "Early printk"
+	depends on DEBUG_LL
+	help
+	  Say Y here if you want to have an early console using the
+	  kernel low-level debugging functions. Add earlyprintk to your
+	  kernel parameters to enable this console.
+
 config DEBUG_ICEDCC
 	bool "Kernel low-level debugging via EmbeddedICE DCC channel"
 	depends on DEBUG_LL

arch/arm/kernel/Makefile

@@ -52,5 +52,6 @@ endif
 
 head-y			:= head$(MMUEXT).o
 obj-$(CONFIG_DEBUG_LL)	+= debug.o
+obj-$(CONFIG_EARLY_PRINTK)	+= early_printk.o
 
 extra-y := $(head-y) init_task.o vmlinux.lds

arch/arm/kernel/early_printk.c

@@ -0,0 +1,57 @@
+/*
+ *  linux/arch/arm/kernel/early_printk.c
+ *
+ *  Copyright (C) 2009 Sascha Hauer <s.hauer@pengutronix.de>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include <linux/kernel.h>
+#include <linux/console.h>
+#include <linux/init.h>
+
+extern void printch(int);
+
+static void early_write(const char *s, unsigned n)
+{
+	while (n-- > 0) {
+		if (*s == '\n')
+			printch('\r');
+		printch(*s);
+		s++;
+	}
+}
+
+static void early_console_write(struct console *con, const char *s, unsigned n)
+{
+	early_write(s, n);
+}
+
+static struct console early_console = {
+	.name =		"earlycon",
+	.write =	early_console_write,
+	.flags =	CON_PRINTBUFFER | CON_BOOT,
+	.index =	-1,
+};
+
+asmlinkage void early_printk(const char *fmt, ...)
+{
+	char buf[512];
+	int n;
+	va_list ap;
+
+	va_start(ap, fmt);
+	n = vscnprintf(buf, sizeof(buf), fmt, ap);
+	early_write(buf, n);
+	va_end(ap);
+}
+
+static int __init setup_early_printk(char *buf)
+{
+	register_console(&early_console);
+	return 0;
+}
+
+early_param("earlyprintk", setup_early_printk);